Debugging hooks

Not everything works first time, and sometimes even when it does work, things don't happen quite as you expected. That's why Juju includes two tools to help you debug charm hooks: The juju debug-hooks command and the juju debug-log command.

The dhx debugging plugin is also available.

The 'debug-hooks' command

The juju debug-hooks command accepts a unit and an optional list of hooks to debug (which must be named individually in a space-delimited list) or no hook names, causing all hooks to be debugged:

juju debug-hooks <service/unit> [hook-name hook-name2 ...]

So for example, imagine you are deploying the mysql charm and you want to only check the db-relation-joined and db-relation-broken hooks:

juju debug-hooks mysql/0 db-relation-joined db-relation-broken

Note: It is possible and often desirable to run debug-hooks on more than one unit at a time. You should open a new terminal window for each.

Running a debug session

When you run the debug-hooks command, Juju creates a slightly customised tmux session on the machine running the requested unit (if you are unfamiliar with tmux, see the primer at the end of this section).

The tmux session will start with window 0. This window does nothing other than keep the session alive (though it can be used, for example, to view the logs for that unit (in /var/log/juju/).

When the first hook event is queued for a hook that is in the list of those to be debugged:

  • a new window will be attached to the tmux session.
  • the bottom left of the status bar will change to indicate the current hook that has been trapped.
  • the prompt will change to indicate the hook related to the window (e.g. mysql/0:db-relation-joined %)
  • the shell will be running in the standard hook environment.
  • additionally, $JUJU_HOOK_NAME is set appropriately. Image showing tmux debug hooks session

To proceed, you should manually execute the hook, or perform whatever other actions you want. At any time during a debug-hooks window 1 session you can run any of the standard hooks (install, start, stop, config-changed, upgrade-charm) however you can not run any of the relation hooks - these should only be run during their respective trapped events.

For example, for the db-relation-joined hook we mentioned above, you could run:


Whilst you are debugging a hook, it is possible that other hooks will be queued to run. Even hooks which are not in your list to debug will be suspended in the queue until you exit your current window. See the special considerations below.

The queue for pending hooks will restart once you exit the window with an exit command.

Note: To allow Juju to continue processing events normally, you must exit the hook execution window with the exit command, otherwise all further events on that unit will be paused indefinitely.

The queue can be halted by exiting with an exit 1 command, which will flag the hook as failed. Juju will revert to its normal behaviour of suspending everything until this error status is resolved, which you can do by issuing the command (from your juju terminal window, not the debugging window) of juju resolved <unit>.

You can finish your debugging session by closing all windows in the tmux session. Make sure to exit appropriately from all hook windows before terminating.

Debugging early hooks

The install, config-changed, and start hooks often execute quite soon after the unit comes up, making it difficult to start a debug-hooks session in time to intercept them. If you're having difficulties, you can temporarily return an error code from your install hook (e.g. add an exit 1 at the end of it), and start your session only when the unit reports an error status . You should then run juju resolved --retry for the affected unit, and go back to the debug-hooks session to interact.

Special considerations

While you're debugging hooks for one unit on a machine, you're blocking execution of all hooks on that machine. This is generally helpful, because you don't want to have to contend with concurrent changes to the machine's software, but you should be aware that multiple debug-hooks sessions for units assigned to the same machine will block one another, and that you can't control relative execution order directly (other than by erroring out of hooks you don't want to run yet, and retrying them later).

The 'debug-log' command

Logs are indispensable when it comes time to troubleshoot. View the logs with the debug-log command. See Viewing logs.

What on earth is tmux?

The Ubuntu images used by Juju also include a slightly customised version of tmux, a terminal multiplexer.

It is very useful when debugging hooks, as new hook events can be opened as windows within tmux, meaning you don't need to deal with so many open windows, or (hopefully) get confused as to where to run commands.

This version runs through a layer called "byobu", which additionally adds some useful status information at the bottom of the screen.

Some useful definitions in the context of tmux:


A running tmux environment, involving one or more windows/panes.


A single screen, possibly containing many panes.


A rectangular section of the current window, where commands are entered.

A session may look something like this:

Image showing byobu and tmux


  1. The default prompt is changed to indicate the machine you are running on, and the hook that you are debugging.
  2. This is a tmux pane, after the screen has been split horizontally.
  3. Pane dividing line. The active pane is the one with the cursor in!
  4. The byobu display shows the OS and version info.
  5. This area lists the currently open windows, the current one is highlighted.
  6. The end part of the status bar indicates uptime, load, processor, memory usage and the current date/time.

Key bindings

All of tmux's special functions can be run by pressing the prefix-key followed by another key or key combination. The default prefix-key for tmux is Control-b, but many users find this an unnecessary stretch. This version uses Control-a as the prefix key, which is a bit easier on the fingers and is also the same combination used by screen, an alternative terminal multiplexer.

There are many key-combinations, not all of which are of use in the task at hand. Here are some of the ones you may find useful:


Shortcut Description
Control-a 0 Switch to window 0 (similarly, any other numbered window).
Control-a c Create a new window.
Control-a n Switch to the next window.
Control-a p Switch to the previous window.
Control-a w Choose a window from a list.


Shortcut Description
Control-a % Split pane vertically.
Control-a | Split pane horizontally.
Control-a o Switch focus to next pane.
Control-a ; Switch focus to the previous pane.
Control-a ! Move current pane to a new window.
Control-a x Kill the current pane.


Shortcut Description
Control-a ? Show currently configured keys.
Control-a : Enter the command prompt (for tmux commands)

You can get more info on tmux and its commands at the relevant Ubuntu manpage entry.

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